1. Field of the Invention
This invention relates to pneumatic radial tires for motor vehicles having a steel cord reinforced structure and a process of far producing the steel cord reinforced structure.
2. Prior Art
Advanced pneumatic radial-ply tires for passenger cars are provided typically with belt layers made up of rubber coated steel cords. Organic fiber cords have also found wide application for the belt portion of the radial tire but have mostly been accompanied with the use of rubber coated steel cords at least for one belt layer to provide increased rigidity.
While steel cords are suitable for use as belt reinforcement material in view of their high rigidity and strength, the cords being steel are susceptible to corrosion attack by water permeating through an external injury in the tire which would lead to separation from the rubber coating in which the cords are embedded and thus pose a durability problem. To solve this problem, there has been proposed an open-twisted type steel cord as disclosed for example in U.S. Pat. Nos. 4,399,853 and 4,718,470 which comprises a 1.times.n strand construction such as 1.times.5, 1.times.4 and 1.times.3 having filament-to-filament spaces filled with rubber. This steel cord structure is characterized by non-uniformity of the cross-sectional shape distribution over the entire length of the cord. FIG. 3 in the accompanying drawings illustrates a typical example of the prior art in which an open-twisted cord is formed with a group of steel filaments all isolated or spaced apart from each other and another group of steel filaments some of which are isolated from one another and the others held partly in physical contact with each other, the two groups alternating one with another over the length of the cord.
Another prior art example is directed to a composite metal and rubber material having a plurality of different cross-sectional shapes for the metal cord. Both prior art cord systems have a common feature of a cross-sectional shape distribution being irregular throughout the length of the cord or in its axial direction. This has a drawback in that strains resulting from compressive deformation of the tire on running tend to be localised inducing a fracture or breakage of the cord.